FR 2 652 866 describes a method of tensioning a multiple-strand cable which ensures a uniform distribution of the tension between all of the strands (see item a) above). In this method, the first strand, referred to as a witness strand, is installed and tensioned first, and is then anchored and provided with a measuring cell which indicates the tension in the witness strand at all times during installation of the cable. A second strand is then installed, progressively tensioned, and anchored at the precise moment that the tension in it is equal to that in the witness strand. The same procedure is followed for the third strand: it is introduced into the cable and then progressively tensioned until its tension is equal to that of the witness strand. This process continues until the last strand has been tensioned and anchored. The tension in the witness strand is then released, the measuring cell is removed, and the witness strand is then tensioned again to the final tension indicated by the cell.
Consequently, each time that a new strand is introduced into the cable, its tension is set relative to that of the witness strand; the two tensions, which are equal at the time the new strand is anchored, remain equal because they vary in the same manner afterward if the relative position of the cable anchors changes:
a) either because of a progressive increase in the tension in the cable as the strands are installed,
b) or because of an external load applied to the structure.
This method, which is known as the isotension method, therefore enables all of the strands of a cable to be installed with an a priori guarantee of uniform distribution of the total force in the cable between all the strands.
It has a number of drawbacks, however.
First of all, the tensions in the new strand and the witness strand remain equal only if the two strands were at the same temperature at the moment of anchoring the new strand. Experience shows that this is not always the case: the witness strand is contained in a sheath exposed to sunlight and its temperature can be several tens of degrees higher than that of the new strand being installed. When the temperatures have equalized, a relative difference in the values of the tensions in the strands is then observed, and can exceed 10%. This sometimes requires a retensioning operation, using the method described, to equalize the tensions in the strands, and this represents additional work.
Moreover, although the prior art method imposes the same tension in all the strands at the time of installing the cable, the problem of adjusting a cable in accordance with the specifications imposed by the design of the structure remain outside the scope of the prior art method.
One approach that might be envisaged consists of using stiffness characteristics of the structure to which the cable is fixed to compute the tension to be applied to the first strand so that at the end of the installation of the strands the tension in the cable reaches a specific total. However, experience shows that this approach is imprecise, because of uncertainties as to the real load on the first strand, such as the real conditions of contact of the sheath, the presence of end tubes temporarily supported by the strand, etc. In practice, this problem is overcome by proceeding in two stages:                the strands are initially installed as previously described at a fraction of the final tension (from 60% to 90%, depending on the project),        appropriate means are then used to compute the stretch to be imparted to all of the strands to achieve the final tension, this computation yielding reliable results since it is reasonable to assume that the weight of the sheath is uniformly distributed between all of the strands; the calculated stretch is then usually applied to the witness strand, after which the other strands are retensioned using the isotension method.        
It is obvious that this two-fold process greatly complicates the work and therefore increases the cost associated with installing and adjusting a cable.
Finally, the prior art method requires the tension in the witness strand to be released at the end of the work, followed by demounting the measuring cell and retensioning the witness strand to the previously measured tension; this also complicates the work.